The commercial value of pressure transducers that incorporate optical fiber technology continues to increase. This increasing value stems from the many and varied applications being found for these devices. For example, because of their ability to provide rapid and accurate responses to pressure changes as well as the ability to manufacture the devices in clean room conditions and from biocompatible materials, fiber optic pressure transducers are being widely used in the medical community.
Broadly speaking, these devices include a pressure responsive member having a first surface exposed to the pressure desired to be measured and a second surface exposed to ambient atmospheric pressure The devices usually also include a housing that holds the pressure responsive member in a predisposed position relative to a housing aperture that allows the pressure responsive member to flex in the direction of least pressure in response to changing pressure differentials across the two surfaces. The housing also holds the distal end of an optical pathway in a predetermined position relative to the housing aperture and the pressure responsive member. The optical pathway includes a transmission path and a return path, which may comprise one or more optical fibers. Often, the pathway will include a reflector disposed at the distal end of the optical pathway in an optical coupling relation between the light transmission path and the light return path.
In operation, a light signal generated by a light source, such as a light emitting diode, is sent through the pathway on the transmission path from its proximal end to its distal end and then back through the pathway on the return path. Pressure differentials across the two surfaces of the pressure responsive member result in the movement of the pressure responsive member, thereby modulating the return light signal by variably obstructing the light signal. This modulation of the light signal results in changes in the intensity of the light returning through the pathway. A control unit receives the returned light signal and then, after appropriate conditioning of the return signal in a manner well known to the art, compares the intensity of the returning light signal with a correlation scheme that has been previously established between the measured intensity of the returned light signal and the pressure sought to be determined by the pressure transducer. This correlation scheme then provides an indication of the sensed pressure.
As noted above, prior to sing these devices they must be calibrated. Calibration of the instrument typically involves recording the modulated light signal returned to the control unit and a corresponding known pressure that is applied to the first surface. The modulated light signal in essence measures the amount of deflection of the pressure responsive member caused by the pressure differential across the first and second surfaces, and therefore also provides an indication of the pressure differential across the member, Normally, the second surface is exposed to a known pressure, most often ambient atmospheric pressure and, thus, the pressure differential measured is with respect to ambient atmospheric pressure. Calibration of the instrument will occur over the expected range of pressures that the transducer will experience.
Various calibration techniques are known to the medical instrument art. One such technique involves construction of a look-up table that matches the modulated light signals on a one-to-one basis with known pressures. During operation of the transducer then, the returned modulated light signal is simply matched against the look-up table with the calibrated pressure, which in turn is provided to an appropriate display for viewing by the attending clinician or physician. While relatively simple in concept and to implement, this technique has the failing that it does not in and of itself eliminate spikes that may occur in the calibration data. Such spikes may be caused by noise or interference of some sort in the signal itself or during the processing of the signal in the control unit. That is, the data, when plotted on a graph of light signal strength versus pressure, should ideally yield a substantially smooth curve. In reality, however, the calibration data frequently includes data points that lie substantially off the ideal curve. Matching a light signal against such a data point will result in a pressure reading that is substantially incorrect.
The requirement of having a large number of fiber optic pressure transducers perform accurately in a wide variety of clinical settings and with control units of varying type leads to a need for tightly controlled manufacturing processes so that the pressure transducers themselves perform within an expected range. As manufacturing tolerances tighten, the number of transducers that must be discarded for failing to meet product specifications increases. In addition to that type of additional cost, the cost of manufacture itself increases in and of itself. Finally, since the overall costs of manufacturing the transducers increases, the cost per transducer increases since the cost of the acceptable transducers must reflect not only the increased cost of the manufacturing process per se, but also the cost of the waste due to the throwaways that do not meet the product specifications.
It would be desirable to have a method of calibration of a fiber optic pressure transducer that reduced the likelihood of false pressure readings and that reduced the cost of manufacturing such transducers by reducing the need for establishing overly exacting manufacturing specifications and tolerances and by reducing the number of discarded units due to their failure to meet those specifications and tolerances.